The present invention relates to a photoconductive imaging device, such as a vidicon camera tube or electrophotographic device, and particularly to a photoconductive imaging device having a solid insulator material on a surface of a body of semiconductor material and forming a blocking contact with the semiconductor material.
The target for a vidicon type camera tube, in general, comprises a photoconductor body having one surface which is exposed to the light pattern being viewed and an opposed surface which is scanned by an electron beam. A major problem in the design of such targets, particularly where the photoconductor is a semiconductor material, is to insure that the scanning electron beam makes blocking contact to the photoconductor, i.e., the electrons do not enter the interior volume of the photoconductor. At the same time, it is necessary that the holes generated in the photoconductor by the viewed light pattern can leave the photoconductor on the scanned side in order to be combined with and be neutralized by the electrons from the scanning beam.
Ordinarily, such blocking contact has been provided either by a high work function material on the scanned surface of the photoconductor which forms a Schottky surface barrier junction, or by a P type conductivity region along the scanned surface of an N type conductivity photoconductor to form a blocking PN junction at the scanned surface. However, in both of these techniques, the surface of the photoconductor becomes laterally too conducting to preserve the charge pattern and must be broken up or structured in some way into an array of individual blocking contacts so as to preserve surface insulation. The need to have a structured surface has the disadvantage that the process required to make such a surface is relatively expensive and defect prone. Further, camera tubes which have employed insulator type layers in non-structured camera tube targets, on semiconductor photoconductive bodies, such as CdSe, have required relatively expensive and complicated processing to avoid, to some extent, undesirable levels of dark current, and/or other important characteristics.
The photoconductor plate of an electrophotographic device is similar to the target of a camera tube and has similar problems.